The following abbreviations and terms are herewith defined, at least some of which are referred to within the following description of the present disclosure.    3GPP 3rd-Generation Partnership Project    AGCH Access Grant Channel    ASIC Application Specific Integrated Circuit    BCCH Broadcast Control Channel    BLER Block Error Rate    BSS Base Station Subsystem    BTS Base Transceiver Station    CC Coverage Class    CCCH Common Control Channel    CN Core Network    CRC Cyclic Redundancy Check    DRX Discontinuous Receive Cycle    EC-GSM Extended Coverage Global System for Mobile Communications    EC-PCH Extended Coverage Paging Channel    EC-SCH Extended Coverage Synchronization Channel    eDRX Extended Discontinuous Receive Cycle    eNB Evolved Node B    DSP Digital Signal Processor    EDGE Enhanced Data rates for GSM Evolution    EGPRS Enhanced General Packet Radio Service    FCCH Frequency Correction Channel    GSM Global System for Mobile Communications    GERAN GSM/EDGE Radio Access Network    GMSK Gaussian Minimum Phase Shift Keying    GP GERAN Plenary    GPC GERAN Plenary Cellular    GPRS General Packet Radio Service    HARQ Hybrid Automatic Repeat Request    HLR Home Location Register    IE Information Element    IMSI International Mobile Subscriber Identity    IoT Internet of Things    LTE Long-Term Evolution    M2M Machine-to-Machine    MBMS Multimedia Broadcast/Multicast Service    MME Mobility Management Entity    MS Mobile Station    MTC Machine Type Communications    NAS Non-Access Stratum    NB Node B    NRI Network Resource Identifier    PCH Paging Channel    PDP Packet Data Protocol    PDN Packet Data Network    PDTCH Packet Data Traffic Channel    PSM Power Saving Mode    P-TMSI Packet Temporary Mobile Subscriber Identity    RA Routing Area    RACH Random Access Channel    RAN Radio Access Network    RAU Routing Area Update    RNC Radio Network Controller    SCH Synchronization Channel    SGSN Serving GPRS Support Node    SI System Information    TAU Tracking Area Update    TDMA Time Division Multiple Access    TS Timeslot    TSC Timing Sequence Code    TSG Technical Specifications Group    UE User Equipment    VLR Visitor Location Register    WCDMA Wideband Code Division Multiple Access    WiMAX Worldwide Interoperability for Microwave Access    Coverage Class (CC): At any point in time a wireless device belongs to a specific uplink/downlink coverage class that corresponds to either the legacy radio interface performance attributes that serve as the reference coverage for legacy cell planning (e.g., a Block Error Rate of 10% after a single radio block transmission on the PDTCH) or a range of radio interface performance attributes degraded compared to the reference coverage (e.g., up to 20 dB lower performance than that of the reference coverage). Coverage class determines the total number of blind transmissions to be used when transmitting/receiving radio blocks. An uplink/downlink coverage class applicable at any point in time can differ between different logical channels. Upon initiating a system access a wireless device determines the uplink/downlink coverage class applicable to the RACH/AGCH based on estimating the number of blind transmissions of a radio block needed by the BSS (radio access network node) receiver/wireless device receiver to experience a BLER (block error rate) of approximately 10%. The BSS determines the uplink/downlink coverage class to be used by a wireless device on the assigned packet channel resources based on estimating the number of blind transmissions of a radio block needed to satisfy a target BLER and considering the number of HARQ retransmissions (of a radio block) that will, on average, be needed for successful reception of a radio block using that target BLER. Note: a wireless device operating with radio interface performance attributes corresponding to the reference coverage (normal coverage) is considered to be in the best coverage class (i.e., coverage class 1) and therefore does not make blind transmissions. In this case, the wireless device may be referred to as a normal coverage wireless device. In contrast, a wireless device operating with radio interface performance attributes corresponding to an extended coverage (i.e., coverage class greater than 1) makes blind transmissions. In this case, the wireless device may be referred to as an extended coverage wireless device.    eDRX cycle: eDiscontinuous reception (eDRX) is a process of a wireless device disabling its ability to receive when it does not expect to receive incoming messages and enabling its ability to receive during a period of reachability when it anticipates the possibility of message reception. For eDRX to operate, the network coordinates with the wireless device regarding when instances of reachability are to occur. The wireless device will therefore wake up and enable message reception only during pre-scheduled periods of reachability. This process reduces the power consumption which extends the battery life of the wireless device and is sometimes called (deep) sleep mode.    Nominal Paging Group: The specific set of EC-PCH blocks a device monitors once per eDRX cycle. The device determines this specific set of EC-PCH blocks using an algorithm that takes into account its IMSI, its eDRX cycle length and its downlink coverage class.
Today it is common to use Serving General Packet Radio Service (GPRS) Support Nodes (SGSNs) in a pool. The benefit of pooling multiple SGSNs is that the service area is larger than the service area of one SGSN. The larger service area results in reduced inter-SGSN RA updates, and it reduces the Home Location Register (HLR) update traffic. Currently, when one of the SGSNs in a pool recovers from an error (e.g., a power failure where an SGSN restarts and therefore loses with respect to the registered wireless devices all knowledge of Packet Temporary Mobile Subscriber Identity (P-TMSI) to International Mobile Subscriber Identity (IMSI) associations), the SGSN may initiate individual paging for each of the currently registered wireless devices using the wireless device's IMSI. For example, the SGSN can request the IMSIs from the HLR and the Visitor Location Register (VLR) to understand which wireless devices are currently registered in the network. Then, with the knowledge of the IMSIs and the cell level Discontinuous Receive (DRX) cycle that is common to all of the wireless devices in the same cell, the SGSN can transmit paging indications to request the wireless devices (e.g., mobile stations (MSs)) to re-register to the network.
Upon reception of paging indications for GPRS services which use the wireless device's IMSI (instead of the wireless device's assigned P-TMSI), the wireless devices will locally deactivate any active packed data protocol (PDP) context(s) and Multimedia Broadcast/Multicast Service (MBMS) context(s), and locally detach from the GPRS. After performing the local detach, the wireless devices perform a GPRS attach or perform a GPRS attach procedure as well as activate PDP context(s) to replace any previously active PDP context(s). This will re-establish a Non-Access-Stratum (NAS) connection between the wireless devices and the core network (SGSNs), and thereby, the wireless devices will receive new P-TMSI assignments. Though this mechanism will work to notify all wireless devices served by a given core network node (SGSN) that the wireless devices need to perform a RAU procedure, it is undesirable because this mechanism will take a significant period of time in order to complete the notification process, during which there may be a substantial reduction in paging capacity due to the transmission of IMSI-based pages (i.e., a P-TMSI-based page requires 32 bits, whereas an IMSI-based page requires 64 bits). As discussed next, this IMSI-based paging mechanism is also a problem when wireless devices implement the new Extended DRX (eDRX) feature.
Extended DRX (eDRX) is a new feature under development that extends the paging cycles up to the hour range while the legacy paging cycle range is in the order of 1-10 seconds (see 3GPP TSG-GERAN Meeting #64 Tdoc GP-140910, entitled “MS Energy Consumption Evaluation, PSM vs. eDRX”—the contents of this document are hereby incorporated herein by reference for all purposes). In the legacy DRX operation, a DRX cycle, defined as the time between instances where the wireless device wakes up to monitor its Paging Channel (PCH), occurs on the order of once every few seconds and the same DRX cycle applies to all wireless devices in the same cell (i.e., the legacy DRX cycle lengths are cell specific). However, when the DRX cycle is extended, the eDRX cycle will be set per wireless device connection, and not per cell. Hence, an eDRX cycle will be determined for a specific wireless device suitable for the wireless device's service, e.g., a wireless device that needs to be contacted every 30 min to see that the wireless device is still operable.
However, when the core network (SGSN) requires a restart such as after a power failure, all information on the wireless device-specific eDRX cycles will be lost, and hence, the wireless devices will wake up according to their last registered eDRX cycle and monitor the PCH. In this case, the core network (SGSN) will no longer know when each wireless device wakes up, and therefore, all wireless devices will effectively become unreachable until the next registration event (e.g., until the next periodic registration event which may occur only once every 24 hours). Accordingly, there is a need to address these problems and other problems associated with a network restart in a pool of core network (CN) nodes.